1. Field of the Invention
The present invention is related generally to thin transparent conducting oxide films and more particularly to thin transparent conducting films of cadmium stannate.
2. Description of Related Art
Transparent conductive oxides (TCOs) are a family of wide band-gap semiconductors which can be doped to relatively high levels and fabricated into thin films exhibiting both good light transmittance and reasonable electrical conductivity. The fundamental absorption edge of TCOs generally lies at about 0.35 .mu.m. Thus, the TCOs transmit light relatively well in a range defined by the short wavelength transition to high absorption and the long wavelength transition to high reflection. TCO layers have been used in applications such as solar cells, heat mirrors, electrochromic windows, and flat panel displays.
Commercially useful TCOs for applications such as flat panel displays have sheet resistances around 20 ohm/square, mobilities around 20-30 cm.sup.2 V.sup.-1 s.sup.-1, and optical transmission of 80-85% over a relatively wide range of visible wavelengths. Coatings useful for architectural purposes, such as heat-reflecting windows, should have high optical transmission throughout the visible light range, but also should exhibit an abrupt decrease in transmission and increase in reflectance at longer infrared wavelengths (1-1.5 .mu.m) to reduce heat-loss effects. For many applications, it would be desirable to obtain sheet resistances as low as 2-5 ohm/sq. and optical transmittances greater than 85 per cent.
TCOs suitable for commercial use must also be easy to produce, durable, smooth, and easy to pattern or etch. Tin oxide (SnO.sub.2) films have been used in commercial TCO applications. However, SnO.sub.2 is difficult to etch chemically to produce desired patterns. In contrast, Cd.sub.2 SnO.sub.4 can be etched easily in hydrochloric acid or hydrofluoric acid.
In 1972, A. J. Nozik reported in Phys. Rev., Sect. B, 6, p. 453, that spinel structured cadmium stannate (Cd.sub.2 SnO.sub.4, or CTO) had an unusually high Hall effect electron mobility. Further investigation by G. Haacke and his coworkers led to development of CTO films such as those described in a report entitled, "Exploratory Development of Transparent Conductor Materials," by G. Haacke, prepared for the Air Force Materials Laboratory in March, 1975, No. AD-A008 783. He found that radio frequency (RF) sputtering onto silica and glass substrates resulted in coatings superior to those deposited by chemical spray or vacuum evaporation or onto other substrates. The deposited films were amorphous unless crystallization was induced by maintaining the substrate at elevated temperature during the sputtering process. Lower substrate temperatures were possible when the sputtering was conducted in pure oxygen than when a combination of oxygen and argon was used. However, glass and silica substrates were heated to temperatures of at least 500.degree. C. during the sputtering operation. Haacke also found that intrinsic dopants, such as interstitial cadmium or oxygen vacancies, increased the film electrical conductivity. Haacke reported CTO films having sheet resistances near 1.5 ohm/square and better than 85% visible transmission when corrected for absorption in the substrate. Film conductivity was maximized by a post-deposition heat treatment for 10 minutes at 650.degree. C. in argon. Haacke's sputtered CTO films contained CdO and CdSnO.sub.3 phases in addition to Cd.sub.2 SnO.sub.4, and the presence of these additional phases reduced free electron mobilities to values less than those observed for amorphous coatings.
For many uses of CTO films, it is desirable to form the film on an inexpensive substrate, such as soda-lime glass. However, soda-lime glass cannot withstand the temperatures required in Haacke's process. Haacke reported that soda-lime glass substrates acquired internal stresses when annealed at 650.degree. C., but could be treated by gradually raising the temperature from 400.degree. C. to 580.degree. C. The resulting films on soda lime glass substrates had 20% lower conductivities than films deposited on other substrates. Haacke stated that electrical sheet resistances in the 5-10 ohm/square were sought, but did not report any electrical properties for soda lime glass samples. Other workers modified the deposition and/or heat treating steps to improve CTO film properties.
Methods developed to date for making high-conductivity, optically transparent CTO films generally comprise two high temperature steps: sputtering and subsequent heat treating. In commercial operations, each high temperature step lengthens the overall production time by adding a cooling period to the process and also increases the energy requirements for the process, thereby increasing costs.
The optical and electrical properties of CTO films produced by Haacke's method can be improved by covering the films with a layer of CdS powder and then heating the films to about 700.degree. C. in an argon atmosphere. However, there are several significant disadvantages with using CdS powder in the CdS/Ar annealing step in the production of CTO films. First, it is difficult to achieve a uniform coating of CdS powder, and therefore also difficult to obtain high quality uniform films after annealing. Second, water and oxygen, inevitably present in the CdS powder, cause stains to develop in CTO films, and it is difficult to maintain the CdS powder, with its relatively large surface area, free of adsorbed water and oxygen. Finally, CdS is toxic and subject to stringent environmental regulations with respect to handling and disposal. The furnace and work area can easily become contaminated with CdS powder, posing a significant environmental hazard.
Thus, there is a need for CTO films with improved optical and electrical properties. There is a further need to obtain uniform, stain-free CTO films. Yet another need is for a method of forming CTO films which minimizes the number of steps which must be conducted at high temperature. A further need is for a method of forming CTO films without subjecting the substrate to high temperatures above 600.degree. C. There is a still further need for a method for forming CTO films with a reduced likelihood of CdS contamination of the work area.
Thus, it is an object of the present invention to provide CTO films formed by a method which results in improved optical and electrical properties of the film.
It is a further object of the present invention to provide a method for forming uniform, stain-free CTO coatings.
It is yet another object of the present invention to provide a method of forming CTO films which decreases the number of steps which must be conducted at high temperature.
It is a yet further object of the present invention to provide a method of forming CTO films without subjecting the substrate to temperatures greater than about 600.degree. C.
It is a still further object of the present invention to provide a method for forming CTO films with a reduced likelihood of CdS contamination of the work area.